En route to what is now the customary catalytic aftertreatment of the exhaust gases from an internal combustion engine, it was initially attempted to reduce exhaust emissions by thermal afterburning. Thermal afterburning allows the unburnt constituents still present in the exhaust gas to be afterburnt by a certain residence time at elevated temperatures. When the engine is running in a rich operating mode, what is known as secondary air injection is required for this purpose. When the engine is running in lean mode, the afterburning is effected by the residual oxygen which is still present in the exhaust gas. The use of catalytic converter technology meant that thermal afterburning was only still required during the warm-up phase of the engine. Thermal afterburning allows the emission of hydrocarbons and carbon monoxides to be abated in the operating phase up until the catalytic converter has reached its operating temperature. Therefore, the use of thermal aftertreatment with secondary air injection during the warm-up phase of the engine in combination with catalytic aftertreatment plays a significant role in complying with strict exhaust emission regulations by significantly shortening the heat-up time required to reach the operating temperature of the catalytic converter.
An internal combustion engine of the generic type and a method of the generic type for monitoring the secondary air injection are known, for example, from international patent application WO 02/052130 A1. This application includes a method and an apparatus for monitoring and determining a mass flow of secondary air which is injected into the exhaust system of an internal combustion engine. In this case, the secondary air is compressed using a compressor in the flow path of the secondary air injection and the mass flow is controlled using throttle valves, control valves and power control of the compressor. In this case, the delivery of secondary air is measured and monitored using a temperature sensor and a measurement principle which makes use of the fact that a temperature rise is recorded in the air compressed by the secondary air compressor. In this context, there is a direct relationship between the temperature rise caused by the compression process and the quantity of secondary air delivered by the compressor. The measured temperature values can therefore be assessed with regard to the delivery capacity of the secondary air compressor and in this way allow open-loop or closed-loop control of the latter. This advantageously makes it possible to dispense with air mass sensors which are otherwise customary. Air mass sensors, for example based on hot-film strain gauges, are expensive and sensitive to soiling.
Working on the basis of the prior art described above, the invention is based on the object of providing an internal combustion engine with an alternative form of monitoring and control of the secondary air injection, in which as far as possible temperature sensors or air mass sensors can be dispensed with.